1. Field of the Invention
The present invention relates generally to nuclear reactors and, more particularly, to a spacer for use in spacing a protective grid from a nozzle of a fuel assembly of a nuclear reactor.
2. Description of the Related Art
Numerous types of nuclear reactors are known and understood in the relevant art. One type of nuclear reactor is a pressurized water reactor which typically includes a plurality of fuel assemblies that are disposed within a jacket through which pressurized water flows. Such water typically flows through the reactor vessel at high velocities, whereby any debris in the water can eventually cause significant wear to the reactor components with which the debris comes into contact.
Each fuel assembly typically includes a plurality of fuel rods that are mounted in a plurality of grids that space the fuel rods from one another and resist vibration of the fuel rods during operation of the reactor. The grids typically include a top grid, a bottom grid, and a plurality of middle grids that are spaced between the top and bottom grids. The grids typically are mounted on elongated thimble tubes that are hollow and can carry instrumentation and/or control rods therein. The fuel rods, grids, and thimble tubes together are mounted between an upper nozzle and a lower nozzle that retain the fuel rods, grids, and thimble tubes in a desired position within the nuclear reactor. The fuel rods, grids, thimble tubes, and nozzles make up the fuel assembly.
Due to the potential that debris may damage components of the fuel assembly, it is known to additionally provide a protective grid that is disposed between the bottom grid and the lower nozzle to protect the fuel rods from debris. It is also known to securely dispose the protective grid against the lower nozzle and to space the protective grid from the lower nozzle in order avoid undesirable contact between the lower nozzle and irregularities on the edges of the assembled straps. Examples of such irregularities would include any types of protruding materials such as welding tabs and other strap material at intersections where the straps out of which the grids are assembled are joined with one another.
All of the grids of the fuel assemblies of the nuclear reactor, including the protective grids, are typically made up of a plurality of straps that are arranged in a grid or lattice pattern and are fastened to one another to define a plurality of cells. The cells include thimble cells and fuel rod cells. The top grid, bottom grid, and middle grids typically are mechanically or otherwise fastened to the thimble tubes which are disposed in the thimble cells. The fuel rods typically are held within the fuel rod cells of the grids, including the protective grid, by a plurality of springs and/or dimples within each fuel rod cell that are formed on the straps of the grids.
The protective grid typically additionally includes spacer members that are welded or otherwise fastened thereon and that receive the ends of the thimble tubes therein and that securely dispose the protective grid against the bottom nozzle when the fuel assembly is assembled. While such known protective grids and spacer members have been generally effective for their intended purposes they have not been without limitation.
Previously known protective grids have employed spacer members in the form of either bent metal inserts or weldments of tubular sleeves with endplugs that have been spot welded within the thimble cells of the protective grid and have been configured to both receive the ends of the thimble tubes therein and space the protective grid from the lower nozzle. Dimensions of such spacer members are critical to ensure proper fit-up with the thimble tube diameter and the thimble cell width. The required close tolerances and the welding or bending operation result in a relatively costly spacer member. It is thus desired to provide a simplified spacer member that does not itself require welding or complicated forming.
Since such sleeves and inserts were disposed within the thimble cells of the protective grid, it was necessary to first fully arrange and interconnect the straps of the protective grid in a first arranging and welding operation, and to thereafter receive the sleeves or inserts within the thimble cells and perform a second welding operation therebetween. Such multiple assembly steps, both of which include arranging and welding operations, have been relatively costly. It is thus desired to provide a protective grid with an integrally welded spacer which is assembled with a single arranging and welding step.
In many fuel assemblies, the thimble cells and the fuel rod cells are of substantially the same size. Nevertheless, some fuel assemblies are designed such that the thimble cells are four times the size of the fuel rod cells. Such enlarged thimble cells are open areas of the grid that are the size of a two-by-two set of fuel rod cells. In known protective grids that employ sleeves or inserts, the sleeves or inserts would desirably be welded against the sides of the thimble cell at the points where the sleeves or inserts contact the straps of the thimble cell. In circumstances where the thimble cells are of the two fuel rod cell by two fuel rod cell size, however, the points of contact between the sleeves or inserts and the straps are areas where other straps terminate. Such contact areas thus are extremely difficult to weld due to such interference by the terminating straps. It is thus desired to provide an improved spacer that can be reliably welded to a protective grid in which the thimble cells thereof are of a two fuel rod cell by two fuel rod cell configuration.
In view of the foregoing, a spacer for use with a protective grid of a fuel assembly of a nuclear reactor is a generally washer-shaped plate of material having an engagement surface and a retention surface opposite one another. The protective grid is formed of a plurality of straps to define a plurality of fuel rod cells and a plurality of thimble cells, with the straps adjacent the thimble cells being notched to receive the spacer therein. The retention surface of the spacer is disposed against the notches, and the engagement surface of the spacer protrudes outwardly from the protective grid. When the engagement surfaces of the spacers are engaged against a lower nozzle of a fuel assembly of a nuclear reactor, the spacers space the protective grid from the lower nozzle. The spacer is integrally welded with the protective grid in a single arranging and welding operation.
An aspect of the present invention is to provide a spacer for use with a protective grid of a fuel assembly of a pressurized water reactor wherein the spacer is integrally welded with the protective grid in a single arranging and welding operation.
Another aspect of the present invention is to provide a spacer that is received in notches formed in the straps of a protective grid of a fuel assembly of a pressurized water reactor.
Another aspect of the present invention is to provide a spacer that has a generally plate-like configuration wherein the spacer can be produced in large quantities with required precision at relatively low cost.
Another aspect of the present invention is to provide a spacer in combination with a protective grid in which the spacer includes an engagement surface that protrudes outwardly from the protective grid to space the protective grid from a lower nozzle of a fuel assembly of a pressurized water reactor.
Another aspect of the present invention is to provide a protective grid in combination with a spacer in which the protective grid is formed with a plurality of thimble cells and fuel rod cells, with each of the thimble cells being four time the size of the fuel rod cells, and in which the spacer can be reliably welded to the straps adjacent the thimble cell.
Another aspect of the present invention is to provide a fuel assembly for a nuclear reactor having a spacer that is interposed between and that spaces a protective grid from a lower nozzle.
Another aspect of the present invention is to provide in combination a grid and a spacer for use in a fuel assembly of a nuclear reactor, in which the fuel assembly includes a nozzle, a thimble tube having an end, and a fastener, the fastener being structured to be cooperably connected with the thimble tube, and in which the general nature of the combination can be stated as including a grid including a plurality of straps interconnected with one another to define at least a first thimble cell, the straps each including an edge, a spacer in the form of a plate formed with a hole extending therethrough, the plate having a substantially planar engagement surface and a substantially planar retention surface opposite one another, the plate being fixedly mounted on the grid with the engagement surface protruding outwardly from the grid, the engagement surface being structured to be disposed against the nozzle to space the grid from the nozzle, the hole being structured to receive at least a portion of the fastener therethrough, and the retention surface being structured to receive the end of the thimble tube against it such that the plate is interposed between the nozzle and the end of the thimble tube when the fastener is received through the hole and is cooperably connected with the thimble tube. At least one of the straps that define the at least first thimble cell may include a notch formed in the edge, with the plate being at least partially disposed in the notch.
Another aspect of the present invention is to provide a fuel assembly for a nuclear reactor, in which the general nature of the fuel assembly can be stated as including a grid including a plurality of straps interconnected with one another to define at least a first thimble cell, the straps each including an edge, the edges being generally aligned along a plane that defines a grid surface of the grid, a first spacer in the form of a first plate having a substantially planar engagement surface and a substantially planar retention surface, the first plate being fixedly mounted on the grid with the engagement surface protruding outwardly from the grid surface, a thimble tube having an end, at least a portion of the thimble tube being received in the at least first thimble cell, a nozzle formed with at least a first opening, and a fastener engaged with the thimble tube, at least a portion of the fastener extending through the at least first opening in the nozzle, at least a portion of the fastener extending through the hole in the first plate, the first plate being interposed between the nozzle and the end of the thimble tube, with the engagement surface being disposed against the nozzle and the retention surface being disposed against the end of the thimble tube such that the grid surface is spaced from the nozzle.
Another aspect of the present invention is to provide a method of assembling a grid with a spacer, the general nature of which can be stated as including the steps of forming notches in a plurality of first straps and a plurality of second straps, each notch including a notch base and at least a first notch side, arranging the first and second straps in a grid pattern to define at least a first thimble cell, each of the first and second straps including an edge, the edges being generally aligned along a plane that defines a grid surface, receiving a spacer in the notches, the spacer being in the form of a plate having a substantially planar engagement surface and a substantially planar retention surface and being formed with a hole extending therethrough, protruding the engagement surface of the spacer outwardly from the grid surface, and fixedly attaching the first and second straps with one another and with the spacer in substantially a single fastening operation.